Patients that underwent anterior cervical discectomy with arthroplasty for a cervical radiculopathy due to a herniated disc from the NECK and PROCON trial were analyzed for HO at 12 and 24 months postoperatively. HO was scored according to the McAfee-Mehren classification. The index ROM was defined by a custom developed image analysis tool, and global cervical ROM was measured by Cobb's angle. Clinical outcome was evaluated by means of the Neck Disability Index and SF-36.

The occurrence of HO was 60% at 1 year, and it increased to 76% at 2-year follow-up. 31% of patients was scored as high grade HO at one-year follow-up, and this percentage increased to 50% at two-year follow-up. Clinical outcome does not correlate to HO grade, and no risk factor for high grade HO could be identified. The ROM at the index level was significantly higher in low grade HO group than those patients with high grade HO, but in 15-38% HO grade does not correspond to ROM.

Heterotopic ossification occurs in three fourths of the patients at two years after surgery, but does not necessarily correspond to clinical outcome, nor loss or preservation of ROM. The McAfee-Mehren classification should be combined with ROM evaluation to properly study HO ¹⁾.